Compounds having an absorption in the visible light region are used as an optical element in optical filters for application to image display devices, such as LCDs, PDPs, EL displays, cathode ray tube displays (CRTs), vacuum fluorescent displays, and field emission displays.
Materials that absorb energy to excite electrons and emit electromagnetic radiation as extra energy when the excited electrons return to the ground state exhibit wavelength conversion performance based on the difference of energy between absorption and emission and have been used as a color (or wavelength) converting colorant in dye or pigment formulations, optical filters, agricultural filters, and the like. In particular, organic compounds of such materials have been studied extensively because they have more easily controllable absorption and emission wavelengths than inorganic compounds. Among them, compounds that emit absorbed energy as fluorescence are called fluorescent colorants. Of the fluorescent colorants those emitting visible fluorescence are of high utility and have found application in displays, lighting equipment, such as fluorescent lamps, biological or medical markers, and so on.
An optical filter containing a color conversion colorant, i.e., a wavelength conversion filter is required to have high light resistance in view of its application. When it is applied to image display devices or solar cells, it is desirable that the wavelength after conversion be in the visible region. A color conversion filter achieving a large Stokes shift has been awaited because of its capability of converting light in the UV region, which is unable to be utilized by known color conversion filters, to visible light.
Patent documents 1 to 3 shown below disclose a naphtholactam derivative as a compound useful for wavelength conversion. According to patent documents 1 to 3, however, it is only necessary for the compound for the applications described to be capable of emitting fluorescence, and there is no mention that the naphtholactam derivative is particularly superior. Patent document 4 below discloses a photoelectric device (solar cell module) using a color converting material.